Data storage devices include physical storage elements, such as hard-disk drives and tape drives that provide capacity for the storage and later retrieval of data. In some applications, these storage elements are configured redundantly to safeguard against the loss and corruption of data stored thereon. One such class of redundant storage devices is referred to generally as disk mirroring. Mirroring refers to redundant storage elements storing the same data and is a common function found in enterprise storage and RAID devices.
In some applications, the mirrored storage elements are substantially identical in both their physical and logical configurations. Thus, replicated data is stored in the same physical allocation on either of the mirrored storage elements. A storage controller is configured to maintain the same information on each of the redundant storage elements. Thus, data written to one of the drives will be written to both. Data read from either drive, however, will not impact the mirrored configuration of the drives. This provides some leeway in selecting which of the redundant storage devices to read from.
The performance of a disk-drive system is largely impacted by three system characteristics, i.e., seek time, latency, and data rate. The seek time relates to the delay incurred in positioning the head above the appropriate track. In the worst case, the seek time is defined by the delay incurred in moving the head between the inner and outer tracks. The latency of the system is the time it takes for the rotation of the disk to bring the desired sector to a position underlying the head. The worst-case latency is defined by the time it takes to complete a single rotation of the disk. Finally, the data rate of the system relates to how quickly data can be read from or written to the disk once the head is positioned above the appropriate sector.
It is well known that efficiencies can be realized in retrieving data stored on mirrored drives. Disk mirroring can allow each disk to be accessed separately for reading purposes. This effectively doubles the speed of disk read access (halves the seek time). This is an important consideration in hardware configurations that frequently access the data on the disk. Thus, while one storage element may be busy responding to one read request, a second storage element is available to respond to another read request.